Summary Epigenetic regulation of vascular functions has been found to play crucial roles in cardiovascular diseases. Vascular endothelial cells (ECs), which are exposed to different flow patterns, regulate vascular homeostasis. Differential epigenetic changes, e.g. histone modifications, caused by different flow patterns regulate EC gene expression profile and hence functional consequences. The coupling of histone phosphorylation, methylation, and acetylation have recently been identified to regulate gene expressions through the distinct chromatin remodeling complexes, which would alter the consequential phenotypic outcome. However, there is a paucity of study in the flow-regulation of histone modifications in vascular cells. We hypothesize that the coupling among epigenetic histone phosphorylation, methylation, and acetylation may serve as a transducing mechanism to regulate EC gene expressions under different patterns of flows. We will develop a directed evolution strategy for the systematic optimization and tuning of FRET biosensors with distinct colors to simultaneously monitor different histone modifications with high sensitivity and specificity. These biosensors will be used to track multiple histone modifications simultaneously in the same live cell and unravel the evolving multiplex landscape of histone modifications under different flows. We will further employ the endonuclease-deficient Cas9 (dCas9), small guide RNAs (sgRNAs) and split FPs to track the dynamics of histone modifications at the specific loci of EC phenotype marker genes. Our epigenetic manipulation system will then be employed to modulate epigenetics at these specific loci and determine their effects on gene expressions and consequent cellular functions in single live cells under different flows. The identified epigenetic profiles will then be modulated in vivo, and the consequent gene expression and phenotypic outcome examined. Four specific aims are proposed: 1) Develop and optimize FRET biosensors to visualize the dynamic histone modifications in single cells, 2) Unravel the spatiotemporal coupling of histone phosphorylation-methylation-acetylation in regulating EC functions under different flows, 3) Establish the roles of locus-specific histone modifications in regulating EC gene expression under flows, 4) Elucidate the effect of histone modifications on gene expression and lesion formation in vivo. The simultaneous tracking of the spatiotemporal dynamics of histone modifications in the nucleus in conjunction with cell proliferation and inflammation in a single live cell will allow the elucidation of the spatiotemporal transducing mechanism in regulating epigenetic modulations and pathophysiological consequences upon the exposure of ECs to hemodynamic cues. The mechanistic insights obtained should allow us to identify the potential molecular targets and facilitate the design of pharmaceutical interventions for pathologic processes. As such, the project should have transformative impact in the field of vascular mechanobiology, particularly related to the molecular regulations of cell cycle and inflammation in mediating the development of atherosclerosis.